Sensors for flow measuring devices are known, which work according to the thermal measuring principle. This measuring principle is based on the cooling of a heated resistance thermometer, subsequently also referred to herein as the heating, or active, sensor element, from which heat is withdrawn by the flowing measured medium. The withdrawn energy is resupplied by increasing the electrical heating current. In this way, a constant temperature difference is maintained between the heating and measuring sensor element, thus the active sensor element, and a reference temperature sensor element, which is referred to subsequently herein also as the measuring or passive sensor element. The greater the mass flow, the more energy is required, in order to maintain this difference. The measured heating current is, as a result thereof, proportional to the mass flow. The thermal measuring principle is well established in processes, in which a product is made from a raw or starting material by the application of chemical, physical or biological procedures and is applied successfully in numerous applications. Especially demanding is the application of the measuring principle in water and other liquids such as e.g. oil, since in such cases the heat transfer and the required heating power are in comparison to gases significantly higher and the relevant material properties are very strongly temperature dependent. In the case of velocities >2.5 m/s, therefore, a flattening of the characteristic curve and therewith a lessened sensitivity can be experienced. Moreover, at smaller separations of the sensor elements from one another, in given cases, crosstalk from the active sensor element to the temperature measuring sensor element can be experienced, e.g. in the case of low velocities <0.2 m/s.
Known are sensors, which are composed of two sensor elements, an active element and a passive element, each of which has a cylindrical sensor cap and protrudes from a base surface of a sensor. Soldered in the end face of the sensor caps are resistance sensors. Due to the cylindrical sensor caps, the sensor elements have good thermal insulation and so no crosstalk is experienced. These sensors exhibit, however, a worse characteristic in water than the earlier mentioned sensors. Thus, in the case of increasing flow, the characteristic curve reaches comparatively rapidly a saturation at a maximum power. Upon reaching this power limit, at greater velocity, heat can no longer be transferred into the flow. Since the characteristic curve represents the measuring range for the flow velocity of the medium, within which a measuring can occur, a fast reaching of saturating means measuring is no longer possible in the case of flows that are too fast.